The widely different properties of edible oils and fats stem from the chemical identity of their fatty acids. If these fatty acids have a chain containing sixteen or more carbon atoms and this chain is fully saturated, the resulting triglycerides have a melting point in excess of 60° C. On the other hand, if the fatty acid carbon chain contains one or more double bonds, the resulting triglycerides have much lower melting points.
Edible oils are natural products, meaning that they have been obtained by the processing of agricultural products. These products may be oilseeds in the case of vegetable oils and fats or they may be of animal origin in the case of, for instance, beef or mutton tallow, lard and fish oil. Both vegetable and animal oils and fats display a wide variety of different fatty acids with respect to chain length, degree of unsaturation, position of the double bonds in the carbon chain and geometrical configuration of the double bonds.
If triglycerides were to contain only a single type of fatty acid, the large number of different fatty acids would already cause edible oils and fats to be a complex mixture of different triglycerides. However, since triglycerides generally contain two or three different types of fatty acids, the number of chemically different triglycerides in edible oils and fats is very high indeed. Accordingly, the crystallization behavior of edible oils and fats is fundamentally different from the crystallization of pure compounds like sugar or citric acid destined for the food industry, compounds like p-xylene or terephthalic acid in the petrochemical industry and especially inorganic salts like sodium chloride or sodium carbonate.
The situation is even more complicated because triglycerides can crystallize in different polymorphs having different crystal morphologies. If they are not too different, chemically different triglycerides can form mixed crystals, which affects their solubility. Moreover, edible oils and fats products used in industry always contain partial glycerides (mono- and diglycerides) and their concentration varies. Some of these partial glycerides retard crystal growth and thereby affect the crystallization process of edible oils, which is therefore far more difficult to control than the great majority of industrial crystallization processes.
Fat crystallization affects a large number of food products and processes. It should, for instance, provide chocolate with a snap upon breaking and it should prevent margarine from oiling out. On the other hand, the fat crystals in ghee should sink to the bottom of the container and leave a clear supernatant. In puff pastry, the fat crystals should provide the product with plasticity; in physically ripened cream, the crystals should facilitate churning; in dry fractionation, the crystals should permit the olein to be separated from the stearin. These various demands can only be met by different crystal morphologies and arriving at these different morphologies necessitates using different crystallization techniques: tempering for chocolate, scraped heat exchangers for margarine, a slow cooling for ghee, patience for cream and a several different processes for the fractionation of edible oils and fats.
These different fractionation processes can be divided into three categories: solvent fractionation, detergent fractionation and dry fractionation. Because the solvents used in the solvent fractionation process are inflammable, building an explosion-proof solvent extraction plant is expensive and the recuperation of the solvents is also an energy intensive process. For that reason, no new solvent fractionation plants are being built. The detergent fractionation process originally attained a better selectivity than the dry fractionation process but since the latter process has been improved considerably with respect to selectivity and olein yield, no new detergent fractionation capacity is being installed either. Consequently, all development effort is currently directed towards the dry fractionation process.
In this process, the edible oil or fat to be fractionated is first completely melted, heated for some time to erase crystal memory and then cooled in a controlled manner. This is commonly done in a vessel comprising an agitator and heat exchange elements but the use of trays moving through a cooling tunnel has also been disclosed, as in EP 0 798 369. Cooling is slow (several hours) and the industry employs different cooling profiles for different fats and oils, so there is a large variation in crystallization time. When the crystallization is observed to have reached a certain stage, the crystallization process is interrupted. This can be by terminating the cooling and feeding the crystallizer contents to a filter, or by transferring the crystal slurry to a holding vessel from where a filtration unit is fed with this crystal slurry and separated into a filtrate which is commonly referred to as olein and a filter cake, the stearin. Such a filtration unit can be a nozzle or conical sieve centrifuge as disclosed in U.S. Pat. No. 4,542,036, a membrane press or a vacuum belt or drum filter.
The performance of this separation unit is highly critical with respect to stearin properties, product yields and the economics of the process. If more olein is retained in the filter cake, the fractionation becomes less selective, the stearin properties deteriorate and the olein yield decreases. This is especially deleterious in multi-stage fractionation processes where these pernicious effects are multiplied. Accordingly, attempts are being made to produce stearin filter cakes with the lowest possible olein content.
Two approaches can be distinguished in these attempts. There is the approach that aims at reducing this olein content by adapting the filtration stage as illustrated by the introduction of the centrifuge or the membrane press, and there is the approach that aims at the formation of crystals with a morphology that facilitates olein release during filtration. However, there is no consensus on which crystal parameters facilitate this release or how to specify them, and even if there were, it would still be unclear how to carry out a crystallization of edible oils and fats that ensures that the specifications are met in a preferably reproducible manner. Moreover, there is no consensus either whether this approach should focus on the crystallization hardware (crystallizer, heat exchange elements and agitator) or its software (temperature profile and control).
At the start of a batch crystallization process, the oil or fat to be crystallized is heated some 10° C. above its melting point so that it is completely melted. If this is done within an agitated vessel fitted with heat exchange elements such as, but not limited to, a coil, double jackets or vertical fins or any arrangements of those elements, this heating can be quite fast since it can tolerate a relatively large temperature difference between the heating medium inside the heat exchange elements and the oil or fat to be heated. If the vessel has been provided with a variable speed agitator, this can also run at its maximum speed and thus increase the heat transfer coefficient.
Cooling, on the other hand, may require a much smaller temperature difference between the heating medium and the oil or fat, especially when the heating medium temperature is below the melting point of the oil or fat since too cold a heat exchange surface may cause crystals to be deposited onto that surface and encrust it so that heat transfer is impeded. Cooling may be fast until the cloud point of the fat has been reached but should then be reduced to values in the order of magnitude of around 10° C. per hour to prevent serious supercooling.
Crystals can only grow in a supersaturated (i.e., supercooled) melt but to start growing, they need a nucleus on which to grow. In nucleation, a distinction is made between homogeneous nucleation and heterogeneous nucleation. In the former process, the triglyceride molecules themselves arrange themselves in such a way that they form an incipient crystal, which can then start to grow. This process requires undercooling by up to 30° C. so that in industrial practice, nucleation is heterogeneous only. This is illustrated by the well-known phenomenon that oleins are more difficult to crystallize since heterogeneous nuclei present in the raw material have been concentrated in the stearin so that there are only few left in the olein.
In addition to the two primary nucleation mechanisms mentioned above, there is also secondary nucleation. According to one theory, secondary nuclei form whenever tiny crystals, embryos, are removed from the crystal surface and exceed the critical size. This requires this surface to be rough and the rate of crystal growth must be so slow that the clusters can diffuse away from the crystal face before they become incorporated in the crystal. According to another theory, clusters of more or less oriented molecules may diffuse away from the growing crystal and some of these may subsequently form a new nucleus. The theories are not mutually exclusive and both theories are in accordance with the observation that strong agitation can lead to the formation of many small crystals that are considered to result from secondary nucleation. If these small crystals are mixed with larger crystals, which started to grow earlier, the resulting filter cake will have a poor permeability and retain substantial amounts of olein.